The applicability of asymptotic expansion method for developing an experimental correlation for multimode heat transfer from planar heat generating elements arranged in tandem inside a vertical channel is reported. Eight experiments are conducted for the selected combinations of modified Rayleigh numbers and surface emissivities of the heat sources and channel walls and a correlation for dimensionless maximum temperature of the heat sources as a function of modified Rayleigh number and emissivities of the heat source and channel walls is developed using the method of asymptotic expansion. The dimensionless maximum temperature evaluated from arbitrary experiments and that predicted from the correlation compare well.